1. Field of the Invention
The present invention relates to an optical fiber having a lens formed at an end thereof for introducing light when the optical fiber is connected to a light source, and more particularly, to an optical fiber of this kind which ensures high-efficiency coupling with a light source (usually, a semiconductor laser).
2. Description of the Related Art
A conventional light-emitting module incorporated in an optical communications system comprises a semiconductor laser serving as a light source, and a lens such as a spherical lens, self-focusing lens or aspherical lens interposed between the semiconductor laser and an optical fiber for converging the laser beam onto the core of the optical fiber. Since the light-emitting module is required to provide high coupling efficiency between the semiconductor laser and the optical fiber, the module is assembled with the optical axes of the semiconductor laser, lens, and optical fiber aligned with each other, so as to achieve maximum coupling power.
The alignment of optical axes of the optical coupling system is, however, complicated, and it is almost impossible to integrate a number of modules into a unit by using a semiconductor laser array, because each optical fiber must be aligned with a corresponding semiconductor laser. Thus, it is difficult to reduce the size and cost of the light-emitting module.
Recently, there has been proposed an optical fiber having a lens directly and integrally formed on an end face thereof, i.e., at a portion of the fiber on which light from the light source is incident (this type of optical fiber is hereinafter called "lensed optical fiber"). In the lensed optical fiber, the fiber end itself functions as a lens. Accordingly, when manufacturing light-emitting modules, the number of required component parts can be reduced, because there is no need to use a light-converging lens, and the number of operations associated with the axis alignment can also be reduced, whereby the cost is cut down.
As shown in FIG. 1, a lensed optical fiber has a lens 2 formed integrally at an end of a fiber body 1 composed of a core 1a and a cladding 1b. Where the optical fiber 1 is a silica glass optical fiber, for example, the lens 2 is formed in the following manner: First, while a portion of the silica glass optical fiber is heated by heating means such as a burner, a tensile force is applied to the fiber in the longitudinal direction thereof, whereby the heated portion extends. When the outer diameter of the heated portion has decreased to a predetermined diameter, the optical fiber is cut at the diameter-reduced portion, and then the cut end is again heated for fusion.
In this heating step, the extreme end 2a of the optical fiber, including the core 1a in the center thereof, becomes spherical in shape due to surface tension, and this spherical end functions as a lens. Thus, the lensed optical fiber 1 has a taper portion 2b extending from the extreme end 2a to an outer peripheral edge which is not affected by heat and having a certain inclination determined by the heating and drawing conditions.
The lensed optical fiber produced in this manner is connected to a semiconductor laser 3, and a laser beam is emitted from a light-emitting surface 3a of the semiconductor laser 3. In this case, the laser beam radiates in conical form. In the laser beam thus emitted, a part incident on the spherical surface 2a at the extremity of the core 1a is propagated through the core 1a, as indicated by arrows p in FIG. 1, and is used for optical communications.
Accordingly, in order to increase the coupling efficiency between the semiconductor laser 3 and the lensed optical fiber, it is necessary that the laser beam emitted from the light-emitting surface 3a of the semiconductor laser 3 be converged and focused on the spherical surface 2a of the core 1a with as high efficiency as possible.
In the lensed optical fiber having the above-described structure, however, the spherical end 2a of the core has a small light convergence area. Therefore, this lensed optical fiber has a problem in that the tolerance or allowable range for axial displacement and angular displacement between the spherical end 2a of the core and the light-emitting surface 3a of the semiconductor laser is extremely limited.
Further, in the case of the lensed optical fiber, the radius of the spherical end 2a of the core should be approximately 5 .mu.m. Accordingly, to increase the coupling efficiency between the semiconductor laser 3 and the optical fiber, the spherical surface 2a must be formed with increased precision. This work, however, is very complicated, in view of the fact that the outer diameter of an optical fiber is about 100 .mu.m at most, and a desired lens shape cannot be formed with satisfactory reproducibility.
Furthermore, when connecting the lensed optical fiber to a semiconductor laser, they must be positioned close to each other such that the distance between the distal end of the fiber and the semiconductor laser is about 10 .mu.m.
Thus, the spherical surface 2a of the core of the lensed optical fiber is positioned as close to the light-emitting surface 3a of the semiconductor laser 3 as possible, as shown in FIG. 1, so that the laser beam may be focused on the spherical surface 2a with high efficiency. During this positioning work, however, the distal end 2a of the lensed optical fiber may collide with the light-emitting surface 3a of the semiconductor laser 3, causing damage to the laser resonance surface or the lens 2, or the laser beam reflected at the spherical surface 2a may enter the semiconductor laser 3, making the emission power of the laser 3 extremely unstable.
To avoid the drawback, if the distance between the distal end of the lensed optical fiber and the semiconductor laser 3 is increased as shown in FIG. 2, the laser beam from the semiconductor laser 3 diverges at a greater angle. Accordingly, major part of the laser beam falls upon and is refracted at the taper portion 2b, and then propagated through the cladding 1b as indicated by arrows q, thus unduly increasing the coupling loss between the semiconductor laser and the lensed optical fiber.
To solve the above problem associated with the conventional lensed optical fiber, the inventors of the present application developed a lensed optical fiber having the structure shown in FIG. 3 (Japanese Patent Application No. 4-348019), corresponding to Japanese Laid-Open Patent Publication No. 6-201946.
This lensed optical fiber has an incident light guide portion at an end portion 4 thereof. The incident light guide portion 4 extends from an end face 4a of the fiber to a distal end 1c of the core 1a and is made of an optical material having a uniform refractive index.
The end face 4a of the incident light guide portion 4 is formed in the shape of a hemisphere having a radius substantially equal to that of the optical fiber 1 or a convexity such as a paraboloid. Thus, when this optical fiber is connected to the semiconductor laser 3, the laser beam emitted in conical form from the light-emitting surface 3a of the semiconductor laser 3 falls upon the convex surface 4a formed at the end of the fiber, is refracted at the convex surface 4a and propagated through the incident light guide portion 4, such that the laser beam is concentrated on the distal end 1c of the core 1a of the optical fiber 1.
The optical fiber 1 is previously formed such that the distal end 1c of the core 1a is located in the vicinity of the focal point of the convex surface; therefore, the amount of light concentrated on the core 1a of the optical fiber is extremely large, compared with the case of the lensed optical fiber shown in FIG. 1.
Namely, this lensed optical fiber permits high coupling efficiency with respect to the semiconductor laser, and the distance between the end face 4a of the fiber and the light-emitting surface 3a of the semiconductor laser 3 can be increased. Accordingly, a number of lensed optical fibers of this type can be integrated into a unit by using a semiconductor laser array.
However, since the end face 4a of the incident light guide portion 4 of the above lensed optical fiber is formed in the shape of a convexity such as a hemisphere, the laser beam reaching the distal end 1c of the core 1a is subject to aberration.
A conventional measure to eliminate the aberration is to form the end of an optical fiber into an aspherical shape such as a hyperboloid, by using a CO.sub.2 laser (cf. H. M. Presby, et al., Appl. Opt., 2692 (1991)).
However, when the optical fiber having an end face processed by this method is connected to a semiconductor laser, the distance between the end face of the fiber and the light-emitting surface of the semiconductor laser must be very short, and thus it cannot be said that this optical fiber is suited for practical use.